(a) Field of the Invention
The present invention pertains to a method and an apparatus of performing an epitaxial growth of Group II-VI compound semiconductor crystals, and more particularly it relates to a method and an apparatus of performing an epitaxial growth of ZnSe crystals from a solution thereof.
(b) Description of the Prior Art
Many of Group II-VI compound semiconductor crystals such as ZnS, ZnSe and CdS have energy band gaps greater than those of Group III-V compound semiconductor crystals, and also they are such crystals that the transition of carriers is of the direct transition type. Especially, a ZnSe crystal has an energy band gap of 2.67 eV at room temperature, and accordingly it is a crystal of which research is being made widely as one of noteworthy materials for obtaining LEDs (light-emitting diodes) having emission spectra in the wavelength region (450-490 nm) of purely blue color, which cannot be realized with Group III-V compound semiconductors.
Many of ZnSe compound semiconductor crystals (hereinafter to be abbreviated as ZnSe crystals) are obtained by relying on a melt growth at a high temperature which is typically represented by the Bridgman method. Most of those ZnSe crystals which are obtained by relying on the prior growth method have usually been of the n-type conductivity having a high resistivity. No satisfactory p-type crystal could have been obtained in the past even by doping a p-type impurity into the crystal during the crystal growth process.
Accordingly, while the useful properties of the ZnSe crystal have been appreciated, the technology has not developed so far as to provide a diode to serve as an LED having a satisfactory pn junction.
The reasons are that, during the crystal growth process, in view of the fact that the vapor pressure of a component element Se is much higher than the vapor pressure of the other component element Zn, there develops a deviation from stoichiometry of the crystal being grown, i.e. there develops Se vacancies due to the shortage of the Se element, which Se vacancies act as donors; or even in case a p-type impurity is doped into the crystal being grown, there develops a complex of the Se vacancies and the doped impurity to serve as donors.
Relatively recently, the present inventor has proposed a very effective crystal growth method which eliminates these causes mentioned above, as disclosed in Japanese Patent Preliminary Publication No. Sho 57-183400 translation filed May 9, 1984 entitled "A method and an apparatus for effecting liquid phase growth of II-VI compounds"; Japanese Patent Application No. Sho 56-161837 entitled "Group II-VI compound semiconductor devices"; and Japanese Patent Application No. Sho 57-115893 entitled "A method of growing ZnSe crystals" which forms the basis of Nishizawa U.S. application Ser. No. 509,053 filed June 29, 1983.
This earlier proposed method is such that a component Se element having a high vapor pressure is used as a solvent in a melt in which is doped a p-type impurity selected from Group Ia elements such as Li, Na and K or from Group Ib elements such as Au, Ag and Cu, and that, throughout the growth process, the vapor of the Zn element having a lower vapor pressure than the Se element is supplied onto the surface of the melt, and also that throughout the growth process there is established a constant temperature difference between the region in which recrystallization takes place and the region of the source crystals.
Thanks to this method, it has become possible to obtain, stably, a bulk crystal of the p-type having a good crystal perfection, which, in turn, has brought about a drastic improvement in the technique of fabricating pn junction semiconductor devices.
For example, a crystal which has been obtained by the above-mentioned technique is rinsed in a Zn solution by keeping the crystal therein at, for example, 1000.degree. C., and thereafter subjecting it to a heat treatment for one hour or more, Zn will diffuse into the crystal from its surface to a depth of several tens of micrometers so that the conductivity type of the crystal which has been of the p-type changes into the n-type upto said depth, and thus a pn junction can be formed. According to this method of forming a pn junction by diffusion, however, there arises a deviation from stoichiometry of the crystal in the vicinity of the crystal surface, bringing about the generation of such defects as point defects, which lowers the crystal perfection of the product. As such, even when there is used a good crystal substrate in this earlier method, there has been the drawback that, when an LED device is fabricated with such a crystal, the resulting device will exhibit a lowered efficiency of light emission.
For the foregoing reasons, the present inventor has earlier proposed, in Japanese Patent Application No. Sho 57-115894 entitled "ZnSe Blue Light Emitting Diode", based on the discovery that, as a method for forming a pn junction while keeping a good crystal condition of the substrate crystal without degrading its good crystal perfection, the crystal growth relying on the epitaxial growth technique is the optimum, and that, especially, it is desirable to use, as a substrate, an n-type ZnSe crystal and to perform an epitaxial growth of a p-type crystal on top of this n-type crystal.
The conventional typical crystal growth apparatus which is used in performing an epitaxial growth of a ZnSe crystal is shown in FIGS. 1(a) and (b). That is, an ampule 5 which is employed for the growth is given a horizontal structure. In a growth region 12 is set a substrate 13 made of a single ZnSe crystal and having, for example, a diameter of about 6 to 8 mm and a thickness of about 500 .mu.m. In the stage prior to growth, the ampule 5 is tilted in such manner as shown in FIG. 1(a), and in this state Se 14 which serves as the solvent and ZnSe source crystals 15 are charged in the source crystal region 16, and subjecting them to an alloying condition by keeping them in the melt state for example at 900.degree. C. for 1 to 3 hours. Thereafter, the ampule 5 is positioned to assume a horizontal posture as shown in FIG. 1(b), so that the substrate 13 is rinsed in the Se melt 14, to thereby perform a crystal growth thereon. In this case, the amount of the ZnSe source crystals which are charged in the Se melt is set to be greater than the amount in which the melt becomes a saturated solution, so that ZnSe source crystals are present in the melt also during the growth process.
Since Se has a density of 4.80 g/cm.sup.3 and ZnSe has a density of 5.98 g/cm.sup.3, ZnSe source crystals will sink into the Se melt. Accordingly, in case the ampule has a vertical type structure, ZnSe crystals will naturally sink downwardly prior to the dissolving of the ZnSe source crystals in such an Se melt as that described above. Accordingly, even when the position of the ampule is changed to the horizontal posture to thereby render the melt to a saturated solution, it will be noted that, when it is intended to bring the ampule into its vertical posture to perform a growth, the remaining ZnSe source crystals will tend to sink downwardly. Therefore, in this particular respect, the growth apparatus which is composed principally of the ampule had better be set horizontal to provide the advantage such that those ZnSe molecules which have dissolved into the Se melt in the source crystal region 16 are transported so as to be supplied, in a constant condition, into the growth region 12.
In this horizontal type growth apparatus, however, it will be noted that, owing to the need for establishing a temperature difference between the growth region 12 and the source crystal region 16 throughout the growth process, it becomes difficult to keep a uniform temperature at the entire surface of the substrate throughout the growth process. Another structural disadvantage is that it is difficult to establish a temperature difference in a vertical direction to the substrate. In this respect, a vertical type growth apparatus structure is desirable.